Control devices exist for many, many systems. A common control device that most people are familiar with is an environmental control device such as an HVAC thermostat which can control of variety of environmental factors (heating, cooling, humidity, ventilation, etc.) in their homes or at other locations. While thermostats have been used for many years, such control devices have until relatively recently been simple analog/mechanical devices employing sensors such as bimetallic strips, etc. combined with mercury tilt switches which directly activate, or deactivate, relays, contactors or other HVAC control devices.
Smart thermostats have now been developed which employ digital processors executing potentially complex software programs to better control environmental factors. These smart thermostats are typically equipped with a variety of sensors (solid state temperature and humidity sensors, etc.) and other information (from remote sensors providing occupancy information and/or remote temperatures, etc. and/or from network-connected servers providing weather conditions and forecasts, etc) which provide inputs to the executing software to control the respective HVAC systems. Such smart thermostats are becoming increasingly popular, both because they typically allow remote control and monitoring (typically via Internet applications) of the operation of the smart thermostat and conditions within the controlled environment, but also because they provide increased user comfort and/or reduce HVAC system energy usage. Examples of such smart thermostats include the Ecobee3 controller manufactured by Ecobee, 250 University Avenue, Suite 400 Toronto, ON Canada, M5H 3E5.
While such smart thermostats are significant improvements over prior thermostats, their design, manufacture and operation pose some unique challenges. For example, HVAC control systems typically require the switching of significant amounts of electrical power to activate air conditioning compressors, circulating fans, etc. While prior art analog/mechanical thermostats could typically handle significant levels of electric power, including various exceptional conditions (faults), in contrast digital control devices are much more susceptible to spikes, over voltages, etc. and yet are expected to provide reliable service for many trouble free years. Further, while the advantages of smart thermostats are obvious, consumer behaviors still require that a smart thermostat be affordable, reasonably small in size and, above all, reliable. Meeting all of these criteria is a difficult task.